The adhesives which have been developed to form the corrugated board or paper board are generally starch base adhesives. The earliest corrugating board adhesive patent, U.S. Pat. No. 2,102,937, discloses an adhesive composition comprising gelatinized starch, ungelatinized starch, alkali and borax in a water base. This adhesive has formed the basis for the manufacture of essentially all corrugated paper board to date. This adhesive is a carrier adhesive, wherein a portion of the starch forms a carrier gelatinized phase and the balance of the starch forms an ungelatinized phase. Under conditions of heat and pressure the ungelatinized starch rapidly gelatinizes, absorbs water and causes a rapid increase in viscosity and adhesion of the starch-adhesive composition. Borax also increases the level of viscosity during final gelatinization.
Corrugated paper board or liner board is commonly prepared by first forming a corrugated element. This element is produced by passing a cellulosic sheet between corrugating rolls to form a substantially sinusoidal or serpentine cross-section in the sheet. A starch-based adhesive is commonly applied to the tips of the thus formed sinusoidal portion called flutes. A non-corrugated or planar cellulosic liner is applied against the adhesive coated flutes of the corrugated elements as the corrugated sheet passes between a corrugating roll and a pressure roll. The resulting paper product having the corrugating medium on one side and the planar liner on another side is called a single-faced element.
The single-faced element may be used in certain applications as a liner or buffer material within a container. More commonly, adhesive is applied to the flute tips of the single-faced element and a second liner sheet is subsequently applied to the adhesive liner in a "double facer" operation. The second liner sheet is exposed to conditions of heat and pressure during its contact with the adhesive. In a so-called "hot-plate" dryer section, a continuously moving belt with a hot-plate or platen surface is used to partially remove water from the adhesive bond line and establish a cured adhesive bond at temperatures of about 300.degree.-400.degree. F.
More recently a number of patents have disclosed a corrugating board adhesive that can be characterized as a conventional starch-based adhesive containing finely divided solid particles of a substantially room temperature insoluble polyvinyl alcohol. The water insoluble polyvinyl alcohol disclosed in these patents becomes soluble at the elevated temperature in a corrugating moisture. A substantially insoluble polyvinyl alcohol is used in these patents since the common belief of the corrugating industry as set forth in Czerwin, U.S. Pat. No. 4,094,718, is that soluble polyvinyl alcohol will gel in the presence of borax containing starch based adhesives. Insoluble polyvinyl alcohol permits the use of the polyvinyl alcohol in starch based adhesives since in storage at ambient temperatures the polyvinyl alcohol, in solid form, cannot interact with starch or borax.
The speed at which corrugating machines can be run is limited by the rate of viscosity increase in the bond line between the liner and corrugated medium. As the viscosity of the adhesive rises under conditions of heat and pressure in the corrugating machine, a "green bond" is formed which holds the components together until the adhesive system forms the final heat-cured corrugated bond.
Research has shown that the use of fully hydrolyzed, insoluble polyvinyl alcohol in a starch based adhesive provides a viscosity increase which is reliant on the rate of solubilization of the highly divided polyvinyl alcohol under the appropriate process conditions. Once the polyvinyl alcohol is in solution, it promotes the requisite increase in adhesive viscosity. Further, the use of other polyvinyl alcohols in corrugating adhesives is shown in Krankkala, U.S. Pat. Nos. 4,600,739, 4,673,698 and 4,677,145.